Ablative materials have been used in a number of applications to protect and insulate objects that are subjected to extreme thermal conditions. More specifically, extreme thermal conditions in aerospace vehicles have been managed using a variety of techniques including insulation and radiant cooling, active cooling, conduction and convective cooling, and by phase change or ablative materials. Generally, ablative materials are applied to the affected aerosurfaces and/or substructure to absorb the radiant and convective heat and to insulate the vehicle from the extreme thermal environment.
Aerospace launch vehicles having solid rocket boosters generate high convective and radiant heat near the base region of main engines. To prevent damage from the high heat, structure near the engines is typically protected with a layer of low temperature ablative (LTA) material. The LTA material generally insulates the structure by absorbing the heat through an ablation process, wherein the LTA material forms a char and thereafter burns for a period of time. During exposure to extreme heating and subsequent ablation, the LTA material may decompose and recede across its surface. The recession is generally due to phase change processes such as melting, sublimation, or chemical reactions including oxidation and combustion. Similarly, the decomposition is due to processes such as pyrolysis, phase changes, or chemical reactions.
The performance of LTA materials is often characterized by “q*” or “heat of ablation,” which is defined as:q*=qdot/mdot; where:                qdot=qhw−qrad; (net heat flux)                    qhw=convective hot wall flux;            qrad=net radiative heat flux; and                        mdot=rate of mass loss.        
In order to adequately protect structure and systems from extreme thermal conditions, LTA materials must have a high heat of ablation in addition to low thermal conduction. Furthermore, LTA materials in aerospace applications typically have a low density in order to minimize weight, and are further able to withstand a variety of flight loads, such as aerodynamic shear forces, in addition to extreme heating.
When LTA materials are exposed to high heat flux and oxygen from the atmosphere, the LTA materials quickly char and begin burning. Once ignited, the LTA materials may continue to burn even after the heat source subsides. Accordingly, effective LTA materials typically form a strong char during the ablation process, which is sufficient to prevent separation of at least a portion of the LTA material from the structure due to aerodynamic forces, thermal shock, and vibrations.
Generally, the char provides increased thermal protection because less LTA material is removed during the ablation process. The char is also porous, lightweight, and has low thermal conductivity to further improve thermal protection. Additionally, radiant heat loss is increased since the char has higher emissivity and can withstand higher temperatures, and the higher temperatures further reduce convective heat gain.
Unfortunately, a critical failure mode of LTA materials is the formation of a weakened char. As the material forms a char and burns during the ablation process, cracks may form in the surface of the LTA materials. The cracks typically increase in size over time and eventually cause the LTA material to fracture and erode away due to aerodynamic forces. Therefore, effective LTA materials must be capable of forming a strong char.
LTA materials are also susceptible to moisture absorption due to their porosity and lightweight. Moisture absorption increases the weight of the LTA material and further contributes to weakened char during the ablation process. Accordingly, a thin layer of sealant or paint, such as Corlar®, is applied over the top of the LTA materials, as a coating, to reduce moisture absorption. Unfortunately, the application of a sealant or paint increases the weight of the ablative composition, and further increases manufacturing cycle time and overall costs.
In addition to LTA materials, intumescent materials have also been used in high heat applications. Intumescent materials, generally defined as materials that swell when heated, have been used extensively as thermal barriers in the chemical and oil industries for fire protection. Unfortunately, intumescent materials have a high density and have been undesirable for use in weight sensitive applications such as in aerospace vehicles. Furthermore, the char that is produced by intumescent materials after being subjected to flames cannot withstand high aerodynamic shear forces.
Accordingly, there remains a need in the art for a lightweight ablative composition and methods of forming ablative structures that reduces the amount of ablation, strengthens the char, and protects the LTA against moisture absorption while improving manufacturability and reducing overall costs.